The present invention relates generally to a method and apparatus for electrically stimulating skeletal muscle, and is particularly concerned with a method and apparatus for controlling the amount of muscle fatigue resulting from such stimulation.
The use of electrical pulses or signals to induce muscle contractions and stimulate muscle movement or exercise is well known in the medical field. This technique is commonly known as neuromuscular electrical stimulation (NMES) and is often used in physical or occupational therapy, for example, to strengthen atrophied muscles or paralyzed limbs. NMES is widely used to exercise muscles that are immobilized for long periods of time as a result of paralysis, various muscular or neurological disorders, or extended periods of bed rest arising from injury, surgery, or illness. Such electrical stimulation is also useful for general exercise of otherwise functional muscles to improve muscle tone and strength.
Typically, in electrical stimulation therapy, an oscillating electrical signal is applied to the muscle at a frequency which stimulates muscular activity in the patient. One problem with such therapy is that it does not take into account individual variations between patients. Thus, the same signal may produce fatigue or overstimulation in one patient whereas the muscles of another patient are insufficiently exercised to produce a beneficial effect.
In my U.S. Pat. No. 4,838,272 a closed loop system is described in which electrical stimulation signals are applied to muscles and the work output by the muscles is determined and compared to a target value. The input stimulation signal is then varied according to the detected muscle work output in order to maximize the amount of work output by the muscles during a treatment period.
One problem with this feedback technique based on work output or muscle tension is that the nature of the tension change during electrical stimulation is not completely predictable and reproducible between individuals. Thus, at a given work output, the muscles of one individual may become over fatigued while another individual receives an optimum workout. Because of the variation in tension or work output between individuals, electrical stimulation systems using this factor as a feedback control must be customized for each individual to produce optimum results.